1. Field of the Invention
This invention relates to a solid-state color imaging device and a process for the preparation of the same.
2. Description of the Prior Arts
There has been heretofore known a color imaging tube provided with a micro-color filter on its image sensing surface which has been conventionally incorporated into a camera for a video tape recording (VTR) system to collect color signals corresponding to a color image.
Recently, a variety of solid-state imaging devices such as CCD, BBD and MOS have been proposed and studied for replacing the conventional imaging tubes. Accordingly, a number of trials for providing a solid-state color imaging device employing a combination of the solid-state imaging device and a micro-color filter have been carried out for the purpose of reducing the dimension of a VTR camera. Such a solid-state color imaging device is now partially put to practical use replacing the conventional color imaging tube.
The solid-state color imaging device is provided on its image sensing surface with a combination of photoelectronic convertor elements, generally called image sensor elements, and flat highly-integrated scanning circuits. On the image sensing surface, there is provided a micro- color filter comprising a plurality of micro-color filter elements (colored resin layers) in a mosaic or striped pattern, each of which is colored with a dye such as red, green or blue, otherwise, cyan, magenta or yellow, to match each of the plurality of image sensor elements.
The micro-color filter is generally provided to the solid-state imaging device by a laminating process or an on-wafer process. The laminating process involves initially forming micro-color filter elements on a transparent support such as glass plate to produce a micro-color filter matching an array of the image sensor elements of the solid-state imaging device to which the micro-color filter is provided, and then thus produced micro-color filter is superposed on the imaging surface of the solid-state imaging device under adhesion. The laminating process accordingly requires so careful arrangement that each colored micro-filter element of the independently produced filter assembly can be placed on the image sensing surface to precisely match each imaging sensor element of the solid-state imaging device.
In contrast, the on-wafer process involves formation of the micro-color filter directly on the imaging surface of the solid-state imaging device. Therefore, the on-wafer process can be additionally incorporated into the production line of the solid-state imaging device, and which makes the production of solid-state color imaging device easier.
The on-wafer process can be carried out by two alternative embodiments: one embodiment involves simultaneous production of numerous solid-state color imaging devices by forming colored micro-filter elements on a wafer where numerous solid-state imaging devices are arranged, so that the array of respective micro-filter element matches the corresponding image sensor elements, and another embodiment (on-chip process) involves the first stage of separating an individual solid-state imaging device (chip) from a wafer containing numerous solid-state imaging devices and the second stage of forming a colored micro-filter on the individual chip. In this specification, the term "on-wafer process" includes both the two alternative embodiments.
A representative process for the preparation of a solid-state color imaging device, that is, a process for forming a micro-color filter comprising at least one colored resin layer on an image sensing surface on a solid-state imaging device can be described as follows:
A solution of a photo-hardening resin is coated on the image sensing surface to form a photo-hardening resin layer, and this resin layer is exposed to a radiation through a mask having windows in the desired pattern to harden the resin layer in a mosaic or striped pattern. A resin of the unhardened portion is then removed by washing with an appropriate solvent. Thus hardened resin layer (substrate layer) in a mosaic or striped pattern is subsequently colored with a dye to prepare a colored resin layer. This procedure is further repeated, if desired, to prepare a plurality of colored resin layers to complete the formation of a micro-color filter on the solid-state imaging device.
As the polymer material for the preparation of the hardened-resin layer (substrate layer), a water-soluble protein such as gelatin is generally employed. However, there arises a problem derived from alkali metal ions contained in the protein. For instance, a gelatin generally contains approx. 100-2000 ppm of sodium ion (Na.sup.+) and approx. 20-200 ppm of potassiun ion (K.sup.+).
As described above, the solid-state imaging device comprises image sensor elements and flat-highly integrated scanning circuits, the solid-state imaging device is easily damaged by contamination with impurities such as dust and alkali metals. The solid-state imaging device damaged by the contamination with such impurities shows poor levels in the predetermined characteristics, and thus the yield of acceptable products are rendered low. For this reason, very careful arrangements ought to be provided to a stage for the provision of the micro-color filter to prevent contamination with such impurities as completely as possible.
Particularly, the contamination with a large amount of alkali metals such as sodium and potassium is liable to bring about certain troubles in the operation of the solid-state imaging device. In order to keep the device from lowering of the reliability, it is desired that the amount of alkali metals introduced into the solid-state color imaging device should be made as little as possible. It is now discovered that a solid-state color imaging device, for instance, a CCD type solid-state color imaging device having approx. 0.5 cm.sup.2 of respective image sensing surface area, should not contain more than 100 ppm of alkali metal ions in one chip. Particularly, the alkali metal ions contained in one chip preferably does not exceed 50 ppm.
A solid-state color imaging device having so reduced alkali metal content, however, is hardly obtained by the use of the aforementioned untreated gelatin having the high alkali metal content. Even if gelatin is treated with lime, an acid, or the like, the so treated gelatin still contains a large amount of alkali metals. For instance, gelatin treated with lime ordinarily contains approx. 150 ppm of alkali metals, and gelatin treated with an acid ordinarily contains approx. 500 ppm of alkali metals. The use of the gelatin having alkali metals of so high content introduces the contamination into the solid-state color imaging device.